Our diversion from the mechanism of firefly flash synchronization to the adaptive significance and evolution of this unique communal behavior, mentioned last year, proved sufficiently engrossing to lead to a full-dress theoretical analysis in a paper now in press. The basic problem is that since synchronization is a group behavior it cannot in itself promote the reproductive fortunes of the individual male, which it must do, according to Darwinian natural selection theory, if the behavior is to be perpetuated genetically. Our suggested solution is that sexual selection by the female is on the basis of male flash intensity, which requires simultaneity of presentation because of immediate refractoriness in the response system; the simultaneity requirement in turn provides built-in protection against out-of-phase mutant "cheaters," making the synchronous behavior a true and rare example of group adaptation.